Machine for producing a fibrous web

ABSTRACT

A gap former for forming a fibrous web from a fibrous suspension supplied via a headbox, having a top fabric ( 3, 28 ) and a carrier or bottom fabric ( 2, 27 ), is defined by the fact that the carrier fabric ( 2, 27 ) has a greater thickness than the top fabric ( 3, 28 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 of German Patent Application No. 10 2004 044 570.2 filed on Sep. 15, 2004, the disclosure of which is expressly incorporated by reference herein in its entirety.

BACKGROUND OF INVENTION

1. Field of the Invention

The invention relates to a gap former for forming a fibrous web from a fibrous suspension supplied via a headbox, having a top fabric and a carrier or bottom fabric.

2. Discussion of Background Information

U.S. Pat. Nos. 5,826,627 and 5,152,326 disclose fabrics which consist of a plurality of woven layers with weft and warp threads, the layers being connected to one another by individual threads. In this case there are different possible ways of connecting the woven layers to one another.

In the case of paper webs produced by means of the known fabrics, there is the problem that the printing quality is not always adequate, which manifests itself, for example, by the lack of printed dots (missing dots) or else by means of an excessively high roughness (PPS roughness). There are also problems with the runnability of the paper web, in particular in the fabric separation region, where it is possible for water to be sprayed from the wet-end clothing running on the top side. Since it also contains fibrous material and ash, the sprayed water produces soiling inside and outside the machine. Drops of water dropping on to the fibrous web led out of the wet end and accessible from above, which drops have been produced by condensation of the water mist, soil the fibrous web and lower its quality.

SUMMARY OF THE INVENTION

The object of the invention is to improve a gap former of the type mentioned as the beginning in such a way that it is also suitable for the production of high-quality SC papers.

According to the invention, in a machine of the type mentioned at the beginning, this object is achieved in that the carrier fabric has a greater thickness than the top fabric.

According to the invention, use is made of fabrics which, as compared with fabrics known from the prior art, cause less misting in the exit gap of the fabrics. This is important in particular at speeds of more than 1550 m/min. By means of the invention, the printing quality of the paper can be improved. A paper web of high quality is produced, which has no or barely any defects even at high production speed.

Advantageous developments of the invention are contained in the subclaims, the description and the drawings.

Likewise, it also proves to be advantageous if the closing is or comprises an SSB fabric (SSB=self stitched binding) and/or comprises a plurality of layers of fabrics.

Advantageously, it is also possible to use clothing which, in the case of the carrier fabric, has a thickness of 0.7 mm or less and, in the case of the top fabric, has a thickness of 0.63 mm or less.

The clothing preferably has a void volume of 400 cm³/m² or less.

The invention also relates to the fact that the clothing can advantageously be used as a carrier fabric and has an open area of 32% or more, a void volume of 400 cm³/m² or less with an air permeability of between 360 and 400 cfm. The air permeability is determined in accordance with the TIP 044-20 Standard by the TAPPI measuring method.

The open area contained in the clothing may be calculated in accordance with the following empirically obtained term: (10−(number of warp threads/cm×diameter of the warp threads [mm])×(10−(number of weft threads/cm×diameter of the weft threads [mm]))

If warp and weft threads have different diameters, in each case the average of the diameters is used.

In order to ensure good dewatering of the fibrous web, there are empty spaces within the clothing. A configuration of the invention according to which the number of empty spaces within the fabric on the side facing the fibrous web is less than or equal to the number of empty spaces on the side facing away from the fibrous web is advantageous.

The invention may be used both in the case of clothing having a fabric and in the case of a membrane reinforced by a fabric.

Advantageously, adjustable and movable foils are set against the top fabric and/or against the bottom or carrier fabric.

The invention also relates to a process for producing a fibrous web, in particular a paper web, which is suitable for gravure printing. In this case, use is made of at least one item of clothing as described above.

In a wet end equipped with the fabrics according to the invention for the top fabric and the bottom fabric, a fibrous web can be produced at a speed of more than 1550 m/min or more.

In the following text, the invention will be explained in more detail using two exemplary embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein:

FIG. 1 depicts a twin wire former of the present invention; and

FIG. 2 depicts another embodiment of a twin wire of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice.

A twin-wire former 1 (FIG. 1) comprises a carrier fabric 2 and a top fabric 3, between which, in a twin-wire section 4, a fibrous web is produced from a fibrous suspension supplied in a headbox 5. The top fabric 3 wraps around a forming roll 6 having a suction zone 7, while the carrier fabric 2 is guided over an opposite deflection roll 8. On the side of the carrier fabric 2, a suction device 9 having two suction zones 10, 11 is arranged in the region of the twin-wire section 4. Foils 12, 13, 14 are located opposite the suction device 9. On the side of the top fabric 3, a suction device 15 having suction zones 16, 17 and 18 is fitted.

The carrier fabric 2 is led over a fabric suction roll 19 having two suction zones 20, 21 at the end of the twin-wire section 4. The carrier fabric 2 then moves past two suction devices 22, 23. The top fabric 3 is led onward over a deflection roll 25.

In order to create a particularly smooth surface of the membrane or the clothing, the latter can be ground off on the surface, in particular on the side facing the paper web.

In another exemplary embodiment (FIG. 2), a fibrous suspension is supplied by a head box 25 to a twin-wire zone 26 which is formed between a carrier fabric 27 and a top fabric 28. The headbox 25 is equipped with a large number of slices 29 arranged beside one another in zones, in order to influence the gap width transverse profile.

On the side of the top fabric 28, a forming roll 31 provided with a suction region 31 is arranged in the region of the inlet gap of the headbox 25.

In the region of the exit of the fibrous web from the twin-wire zone 26, the carrier fabric 27 runs over a suction roll 34 equipped with stationary suction zones 32, 33. The fabrics 27, 28 in each case run over deflection rolls 35, 36, 37, 38 and, respectively, 39, 40, 41. Movable and adjustable forming foils 43 are arranged opposite the suction box 42. In the twin-wire zone 26 there is a further suction box 44. Outside the twin-wire zone 26, a further suction box 45 is set against the carrier fabric 27.

For the clothing 2, 3; and 27, 28, it is also possible to use membranes which, in particular, are reinforced by fabrics.

It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to an exemplary embodiment, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims. 

1. A gap former for forming a fibrous web from a fibrous suspension supplied via a headbox, comprising a top fabric and a carrier or bottom fabric, wherein the carrier fabric has a greater thickness than the top fabric.
 2. The gap former of claim 1, wherein the top fabric has a thickness of 0.63 mm or less.
 3. The gap former of claim 2, wherein the top fabric has a void volume of 400 cm³/m² or less.
 4. The gap former of claim 1, wherein the carrier fabric has a thickness of 0.77 mm or less.
 5. The gap former of claim 4, wherein the carrier fabric has a void volume of 400 cm³/m² or less.
 6. The gap former of claim 4, wherein the carrier fabric has an open area of 32% or more.
 7. The gap former of claim 4, wherein the carrier fabric has an air permeability of between 360 and 400 cfm.
 8. The gap former of claim 1, wherein the number of empty spaces within at least on of the carrier fabric and the top fabric on a side facing the fibrous web is less than or equal to the number of empty spaces on a side facing away from the fibrous web.
 9. The gap former claim 1, wherein adjustable and movable foils are set against at least one of the top fabric and the bottom or carrier fabric.
 10. A process for producing a fibrous web, in particular a paper web, which is suitable for gravure printing, having at least one of a top fabric and a carrier fabric of claim 1, wherein the fibrous web is produced at a speed of more than 1550 m/min. 